The present invention relates to a semiconductor laser device which is used instead of a gas laser or a solid laser, or a semiconductor laser device which is used as a light source for a laser printer or for an optical disk recording apparatus.
Conventionally, in a semiconductor laser device of a double hetero-structure, as shown in FIG. 5, clad layers 2 holding an active layer 1 between them are respectively formed of a material which has a greater band gap than the active layer 1 and also has a lower refractive index than the active layer 1. Thus, the clad layers 2 act to confine both the light and carriers in the active layer 1.
Also, in a semiconductor laser device of an SCH structure (Separate Confinement Hetero-structure), as shown in FIG. 6, there are provided on the two sides of an active layer 3 through confinement layers 4, each of which has a greater band gap than the active layer 3, clad layers 5 each of which has a lower refractive index and a greater band gap than the confinement layer 4. The clad layers 5 act to confine both the light and carriers in the confinement layers 4. The carriers are further shut in the active layer 3 and emit light. Here, the semiconductor laser of an SCH structure includes a semiconductor laser of a so called well structure having an active layer formed of a thin film which is so thin that a quantum size effect can be obtained.
In order to improve the above SCH structure, there is proposed a structure in which there are provided barrier layers respectively between an active layer and confinement layers and adjacent to the active layer to thereby improve the confinement of carriers (see Japanese Patent Publication No. Hei. 3-290984). However, in this structure as well, each of the clad layers is formed of a material which has a greater band gap and a lower refractive index than the confinement layer and acts to confine the light and carriers in the confinement layers.
As mentioned above, each of the clad layers of the conventional semiconductor laser is formed of a material which has a greater band gap and a lower refractive index than the active layer and confinement layers and performs a function to confine both the light and carriers.
However, the above-mentioned conventional clad layer has the following problems.
In the case of a semiconductor laser which is formed of an AlGaInP system material on a GaAs single crystal substrate, the clad layer is normally 1 to 1.5 .mu.m in thickness and, therefore, the composition y of (Al.sub.x Ga.sub.1-x)y In.sub.1-y P mixed crystal must be lattice matched accurately to the substrate. Otherwise, a large number of defects such as dislocations and the like will occur in the crystal to produce slight level differences called as cross-hatch on the surface of the crystal, which extremely deteriorates the light emitting property of the laser. However, it is not easy to match accurately and grow the composition y of (Al.sub.x Ga.sub.1-x)y In.sub.1-y P mixed crystal according to a molecular beam epitaxy method or the like. Also, in the case of the crystal growth for the quantum well laser, the compositions of the clad layers, confinement layers and active layer must be matched in such a manner that, in (Al.sub.x Ga.sub.1-x)y In.sub.1-y P, while the temperatures of the raw material cells are varied to keep y at 0.5, x must be changed accurately, which makes it difficult to manufacture the compositions of the above layers.
Also, in an AlGaInP system materia, the mobility of the electron and positive hole thereof is small, that is, one half or less than that of an AlGaAs system crystal. Since heat generation resulting from current injection to raise the temperature of the laser and lower the oscillation efficiency thereof is almost proportional to the electric resistance (.varies.1/(mobility.times.doping density)) of the clad layer, it is necessary to dope P-type impurities to (Al.sub.x Ga.sub.1-x).sub.0.5 In.sub.0.5 P crystal (x .gtoreq.0.8), which is used for the clad layer, at a relatively high density. In the growth of an AlGaInP system crystal according to an MOCVD method, although there is given a report that a high density doping has been accomplished by using Mg for P-type impurities, there are still left many problems to be solved in manufacturing.
Further, the material that has the greatest band gap in the AlGaInP system crystal to be lattice matched to the GaAs substrate is Al.sub.0.5 In.sub.0.5 P. When each of clad layers is formed of Al.sub.0.5 In.sub.0.5 P and an active layer is formed of Ga.sub.0.5 In.sub.0.5 P, then there is obtained a small band gap difference, that is, about 0.45 eV. Due to this, a carrier is easy to overflow firstly in the active layer, which lowers the oscillation efficiency of the laser as well as the temperature characteristic of the laser. As long as a clad layer is manufactured which is lattice matched to a substrate, it is impossible to expand a band gap more than that of the currently used material and, as means for solving this, there has been only attempted a method to produce a complicated structure such as a multiple quantum barrier structure or the like. In this method, there is used a superlattice which is formed of a super-thin film composed of several atomic layers but it is difficult to produce the superlattice.